What is the certificate in Holm Security OnPrem?

The system comes with an out-of-box self-signed certificate. You can choose to create a valid certificate, which
can be uploaded and applied to all running services.
The certificate should be in PEM format with one certificate (.crt) and one key (.key), created with at least 2048 bits. 
Certificates signed with their own Certificate Authority (CA) must have the CA bundled into the .crt file and the server certificate file. If an intermediate certificate is used, it must also be bundled into the .crt file.  

You can read more about uploading your own certificate in this article.